1. Field of the Invention
The present invention relates to poppet valves made of ceramics, particularly of the kind used as intake and exhaust valves for an internal combustion engine.
2. Description of the Prior Art
In order to attain a high speed and high output of an internal combustion engine, it has recently been proposed to form the intake and exhaust valves from ceramics as silicon nitride, sialon, etc. which is excellent in rated strength, heat resisting property and sliding property.
In case of forming the intake and exhaust valves from ceramics, it is important from the point of view of the number of manufacturing processes and manufacturing cost to reduce the valve portions to be ground as much as possible. Due to this, it is considered to utilize the valve of which valve head inclined surface extending between the valve stem and the valve seat contacting portion is not ground but merely baked.
However, the unground or merely baked surface of the ceramic article is lower in strength as compared with the ground surface and therefore inferior in reliability to same. Furthermore, the valve head inclined surface is used under both mechanically and thermally servere conditions.
For this reason, the intake and exhaust valves made of ceramics have not made the best use of the lightweight feature and not fully used for attaining the high output of the internal combustion engine.
In order to solve the above described problem, it is considered to make the valve head thicker. This however increases not only the weight of the valve of itself but the resistance in feeding and scavenging the engine, thus reducing the output of the engine.